To determine the kinetics and mechanism of biochemical, structural and ionic events associated with visual excitation and adaptation in rod and cone outer segments using spectroscopic and selective electrode techniques, to construct a model of how these events conspire to produce "receptoral" vision and to test this model against the known kinetics and light dependent behavior of the receptor potential. Our attention is focused principally on control of phosphodiesterase, cyclase, kinase and ATP'ase activities as well as on disk membrane lateral diffusion, structure change potential and ionic conductance control by light.